Let's leap right into things, and see what Tyler wants to know, shall we?
Question: If the Ashbringer is really the Light's answer to Frostmourne, then why isn't anyone trying to use it, or destroy it? Is it because that Tirion Fordring has it? If he does, in WotLK then, he needs our help to weaken the Lich King and let him use it?
Answer: Nobody is using the Ashbringer right now because nobody has the Ashbringer in its true, Light-blessed form. Last we knew, the Ashbringer was still in Highlord Mograine's hands in a Corrupted form. The latest Know Your Lore covers the events leading up to that in a little more detail. Currently, we either need to purify The Ashbringer or forge a new one to be able to use it against the Lich King.